Generally, methods of estimating road slope are classified into methods using a driving torque and methods of using a gravitational acceleration sensor.
A load of a vehicle changes depending on road slope, so an acceleration rate of a vehicle speed regarding the driving torque is changed according to road slope. Thus, the method of estimating road slope using the driving torque estimates road slope by using a difference of the increase rate of the vehicle speed. The method of estimating road slope using the driving torque can estimate road slope without an additional sensor. However, the method of estimating road slope by using the driving torque cannot accurately estimate road slope due to change of the driving torque. Thus, an excessive error of road slope estimation occurs due to changes of the driving torque. Moreover, the method of estimating road slope using the driving torque cannot distinguish a load of road slope from a load of carrying freight or towing.
On the other hand, the method of estimating road slope using the gravitational acceleration sensor involves detecting a longitudinal acceleration when the vehicle is located on a slope. Thus, the method of estimating road slope using the gravitational acceleration sensor involves calculating a pitching slope of the vehicle by comparing the longitudinal acceleration with the increase rate of the vehicle speed. Since the pitching slope is road slope if wheels of the vehicle have a fixed height, the method of estimating road slope using the gravitational acceleration sensor can estimate road slope regardless of the driving torque. Also, the method of estimating road slope using the gravitational acceleration sensor can estimate road slope even though the vehicle carries freight or is towed.
The method of estimating road slope using the gravitational acceleration sensor has high accuracy and fast responsiveness compared to the method of estimating road slope using the driving torque. However, when driving resistance is increased due to towing or carrying freight, the method of estimating road slope using the gravitational acceleration sensor may not reflect the increased driving resistance. On the contrary, the method of estimating road slope using the driving torque can compensate the driving resistance by estimating road slope as an upward slope.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.